Fig. 20. (a) Conversion processes from chitosan powders to carbon quantum dots (CQDs) by a hydrothermal method and images of nitrogen-doped carbon quantum dot (N-CQD) aqueous solutions synthesized at different heating times under UV light irradiation. (b) The energy level of N-CQDs with different heating times. (c) Energy level distribution and charge transfer processes at the interface. (d) Characteristic photocurrent density–voltage (J–V) curves for N-CQDs sensitized solar cells under simulated sunlight (AM1.5, 100 mW cm⁻²) (this figure has been adapted/reproduced from ref. 140 with permission from MDPI, Copyright 2020).